Myasthenia gravis is a chronic condition that causes muscles to tire and weaken easily. For example, if you have myasthenia gravis, you may notice that during a meal, your jaw muscles become tired and weak, interfering with your ability to chew food. After you have rested for a little while, the muscles may become strong again, allowing you to resume eating.
This waxing-and-waning weakness of muscles, worsening with use and improving with rest, is a hallmark of this particular disease. There typically are periods when you may notice more symptoms (called an exacerbation), interspersed with periods when your symptoms decrease or disappear (remission).
The disease most commonly affects muscles that control eye and eyelid movement, so the first symptoms you notice may be eyelid drooping and/or blurred or doubled vision. Many myasthenia gravis patients start out with this "ocular myasthenia." The majority will go on to develop weakness in other muscle groups within one or two years.
Myasthenia gravis affecting multiple muscle groups throughout the body is called generalized myasthenia gravis. Other common muscle groups that are affected may make it difficult for you to chew, swallow, smile, shrug, lift your arm up, grip, rise to a stand, or walk up stairs. When the muscles necessary for breathing are affected, a patient is said to be in myasthenic crisis. This is a life-threatening situation.
Though anyone can develop myasthenia gravis, those most likely to do so are women between age 20 and 40 or men between 50 and 70. If a woman with myasthenia gravis gives birth, the baby may have some temporary, but potentially life-threatening, muscle weakness (neonatal myasthenia) because of antibodies that have transferred from the mother's bloodstream. Typically, during the baby's first weeks of life, the antibodies are cleared from the baby's circulation and the baby develops normal muscle tone and strength.
What Causes Myasthenia Gravis?
Under normal conditions, your nerves direct your muscles to work by sending a message through an area called a receptor. The chemical that delivers the message is called acetylcholine. When acetylcholine binds to a nerve receptor, your muscle knows to contract. In myasthenia gravis, you have fewer acetylcholine receptors than you need.
Myasthenia gravis is considered to be an autoimmune disorder. In an autoimmune disease, some of your body's antibodies (cells in your body that are supposed to be programmed to fight foreign invaders such as bacteria, viruses, or fungi) mistake a part of your own body as foreign, resulting in its destruction. In the case of myasthenia gravis, your antibodies attack and destroy the acetylcholine receptors needed for muscle contraction.
No one knows exactly what causes your body to begin producing the antibodies that destroy acetylcholine receptors. In some cases, the process seems to be related to the thymus gland, which helps produce antibodies.
About 15% of all myasthenia gravis patients are found to have a thymoma, a tumor of the thymus. Although most thymomas are benign, the thymus is usually removed (thymectomy) to prevent the potential spread of cancer. In fact, thymectomy seems to improve symptoms of myasthenia gravis in some patients, even if no tumor is present.